A teenager has admitted plotting a 9/11 anniversary bomb attack - possibly targeting an Elton John concert in Hyde Park.
Haroon Syed, 19, was caught chatting online with a British Security Service officer who posed as a fellow extremist willing to help him source weapons for the attack.
Syed, from Hounslow, west London, pleaded guilty at the Old Bailey to a charge of preparation of terrorist acts between April and September last year after a last-ditch attempt to get his case thrown out failed.
The court heard how Syed - whose brother was jailed last year over an IS-inspired terror plot - tried to get weapons online, including a bomb vest or explosives.
He also searched the internet for Islamic State, past terrorist attacks, and potential attack locations, including Oxford Circus and an Elton John concert in Hyde Park on 11 September last year.
When police swooped to arrest him at his home on 8 September and demanded the password to his mobile phone, he told them: "Yeah I.S.I.S - you like that?"
At an earlier hearing, the court heard how key evidence came from Syed's mobile phone and social media messages to fake contact Abu Yusuf.
A conversation in a Slough Costa Coffee with an officer posing as Yusuf was taped and Syed eventually arranged to pick up a £150 nail bomb in the first week of September.
In legal papers prepared for his defence, Syed was described as "highly vulnerable due to family history, lack of education, addiction to violent online games and the arrest and imprisonment of his brother".
His defence team claimed he was groomed by radicals online but he never intended to carry out an attack as his chats with the agent were a "fantasy to see how far it would go".
Syed entered his guilty plea after an unsuccessful attempt to either get the case thrown out, or exclude the key evidence from the online chat.
His lawyer Mark Summers QC argued he should have been given help by the Prevent deradicalisation group rather than steered in the opposite direction.
Judge Michael Topolski QC adjourned sentencing until 8 June for a probation and psychiatric report to be done. He said Syed faced a "discretionary life sentence".
Last June, Syed's brother Nadir, 23, was jailed for life with a minimum term of 15 years after he was found guilty of plotting to carry out a Lee Rigby-style beheading around Remembrance Sunday in 2014.